[volt-nuts] monitoring LTZ1000 chip temperature

[Bruce Griffiths]

In order to understand the dynamics of the LTZ1000 temperature 
regulation loop better in order to estimate the chip temperature 
response due to ambient temperature fluctuations, I am attempting to 
produce a model of the thermal impedance of the chip by fitting a Foster 
RC ladder (later transforming this to the equivalent Cauer filter) to 
the transient thermal impedance charateristics of the LTZ1000.
The datasheet curves for the transient thermal impedance for the LTZ1000 
are inconsistent (they indicate a junction to ambient thermal resistance 
of around 170C/W) with the tabulated thermal resistance (unless this is 
just the junction to case thermal resistance - the datasheet doesnt 
actually specify which thermal resistance junction to case or junction 
to ambient is specified.)
Has anyone obtained transient cooling curves for the LTZ1000 to compare 
with the datasheet curves?
Even a measurement of the junction to ambient temperature difference as 
a function of heater dissipation would be useful.

Whilst monitoring the chip temperature when the heater is driven with a 
fixed power as used when measuring the transient thermal impedance is 
easily done by measuring the Vbe of the temperature sensing transistor 
Q2, monitoring the temperature of the LTZ1000 chip is a little tricky 
once the temperature regulation loop is closed as the temperature 
sensing transistor Q2 can no longer be used to sense chip temperature 
fluctuations.

However the base emitter voltage of Q2 could perhaps be used for this.

Bruce